1. Field of the Invention
This invention relates to electrochromic display panels and more particularly to such panels having a matrix of coincident selection display elements that are periodically refreshed in a matrix selection mode of operation.
2. Description of the Prior Art
It is well known that certain types of materials exhibit an electrochromic characteristic. That is, when coupled between an anode and a cathode they exhibit a change of color at the anode or cathode or both depending upon the electrical energization of the anode relative to the cathode. It is known to provide an electrochromic display in which a plurality of separate segments or display elements are individually energized to provide a desired display characteristic. One such arrangement is disclosed in U.S. Pat. No. 4,008,950 to Chapman et al. However, such arrangements are relatively inconvenient for displays having a large number of display elements because individual control must be maintained over the energization of each display element. Furthermore, it is quite expensive to provide individual cavities for each display element, fill the cavities with an electrochromatic fluid and then seal the cavities.
An arrangement which avoids the necessity of separately filling and sealing individual cavities for each display element is disclosed in U.S. Pat. No. 3,864,589 to Schoot et al. In this arrangement an elongated horizontally extending electrochromic fluid cavity is provided for each of a plurality of electrodes. A plurality of vertically oriented, horizontally spaced individual display element cavities extend in communication with each elongated fluid cavity and orthogonal column electrodes are provided to form a matrix display in which individual electrochromic filled cavities extend between a matrix of row and column electrodes but with each of the individual cavities extending into communication with the elongated horizontally extending fluid cavity so that all individual cavities can be filled by filling a small number of elongated row cavities. This arrangement thus provides both a degree of electrical isolation of the individual display element cavities plus a simplified fluid filling procedure. This arrangement also provides coincident matrix selection of the individual display elements to reduce the complexity of the electrical drive circuit by eliminating the need for a separate individually controlled electrical connection for each display element.
Severe problems have heretofore limited practical applications of matrix addressed electrochromic displays. One is image diffusion and another is cross-talk or the effect which one display element has upon another. The patent to Schoot et al illustrates the two techniques which are most commonly used for minimizing these effects. These are physical isolation of the individual display element cells and use of an electrochromic chemical system in which the colored species deposits out on the electrode. While the use of a horizontally extending pool of electrochromic material as taught by Schoot et al reduces the manufacturing cost, the partial isolation used therein still maintains the manufacturing cost relatively high. The use of solid colored materials limits the choice of the chemical systems which can be used and degrades the speed with which the cells can be colored or erased. Furthermore, in addition to the direct conductivity of the electrochromic fluid between cells, another source of cross-talk is interaction between cells via the electrodes associated with the matrix. An electrically isolated colored display cell in general has a different electrochemical potential difference across it than a clear cell. When these cells are connected together the colored cell tends to drive or color the clear cell. While eliminating the conductivity between cells, the Schoot et al arrangement still suffers from communication between the cells via the electrodes.